The present invention originates from the detection used for example in a thermal camera for analyzing landscapes in real time, for example integrated in an imagery or watching system.
1. Field of the Invention
An infrared thermal camera comprises in the first place, in the focal plane of its optical system, a detection wafer containing a mosaic of several hundred even a thousand infrared photodiodes. Considering the high number of diodes, their connection to an external processing circuit by conventional means is excluded. The detection wafer is therefore superimposed and connected to a pre-processing wafer also disposed, or practically so, in the focal plane--the two wafers are thus hybridized--the pre-processing wafer containing charge transfer circuits, namely and essentially input circuits associated with photodiodes for integrating their output currents, pre-amplification filtering and multiplexing devices and an output circuit, also forming an interface between the pre-processing wafer and a separate wafer for subsequent processing. The connections between the wafers may be of the beam-lead type.
To optimize the performances, namely to minimize the parasite noises, the detection wafer must operate at low temperature. The whole of the focal plane formed by the detection and pre-processing wafers is therefore disposed inside a cryogenic enclosure, or cryostat, associated with a cooling system.
For uniform illumination or background, the amounts of charges vary from one line of detectors to another, which contributes to creating the phenomenon of false contrast, or fixed spatial noise, and which deteriorates the quality of the images. This noise in fact originates in the dispersion of the currents delivered by the different detectors because of the variations of
cut-off wavelengths, PA0 dark currents, PA0 impedances, PA0 quantum yields and sensitive surfaces of the detectors, PA0 biassing voltages.
To avoid saturation of the processing circuits by this phenomenon and so that the quality of the images obtained is not too deteriorated, a certain dynamics is necessary then with respect to the noise level, to which must be added the dynamics proper to the detection signals which depend on the temperature differences considered.
The scene dynamics to be used in the circuits of the pre-processing wafer is one of the fundamental parameters which govern their design. These dynamics are defined by the ratio between the peak to peak deviation of the signal and the effective noise of the scene background.
Technology however does not allow these dynamics to be extended at will. It is necessarily limited and the result is, besides a limitation of the usable temperature deviation, an inevitable saturation phenomenon. Although, in the field of imagery, saturation of the circuit is not too prejudicial, it is not the same when it is desired to detect objects whose temperature is very much greater than that of the scene background. In the range of saturating temperatures, two hot points of respectively different temperatures cannot however be differentiated.
2. Description of the Prior Art
A wafer is already known, particularly from the documents EP-A-0 082 616 and EP-A-0 149 948, for preprocessing the output currents of detection diodes subjected to thermal radiation, comprising an input stage for integrating the output currents of the diodes, an output stage delivering a signal related to the incident radiation by a transfer function and means for non linearly compressing the transfer function of the wafer.
Non linear compression of the transfer function, which makes its representative curve concave towards the axis of the abscissa, that of the radiation values, of the temperatures or of the current delivered by the detection diode causes, in the curved portion alone of the high signals, compression of the dynamics and, especially, displacement towards the high values of the saturation range, which has undeniably an advantage. The low signals are not affected, the value of the integrated background noise is not affected either, so that the thermal sensitivity is not degraded.
The loss of resolution towards the hot points, due to lowering of the transfer curve with the wafer of the above mentioned documents, is not prejudicial, as long as it is sufficient to detect these hot points with an accuracy of only a few degrees.
It will be noted that compression of the transfer function is distinguished from compression by variation of the integration time. This latter is a linear compression, namely the signal varies proportionally to the integration time whereas the noise varies proportionally to the square root of this time; such linear compression adversely affects the strong and weak signals in the same way and so degrades the thermal sensitivity.
The wafer of the type mentioned above is however not fully satisfactory. That of the document EP-A-0 082 616 for example comprises a cascade of wells which cannot ensure the two clipping and shunt-compression functions, this disadvantage appearing more clearly after introduction of the invention of the present application. Furthermore the notions of skimming of the charges and or removing the skimmed charges are also totally absent. As for the wafer of document EP-A-0 149 948, with its additional storage means controlled by variable biassing voltages, it has the same drawbacks as the preceding ones.